The researches on organic solar cell began in 1959. It has a structure of monocrystalline anthracene sandwiched between two cells, the open-circuit voltage of the device is 200 mV, but the photoelectric conversion efficiency is very low. Dr. Deng Qingyun reported a solar cell with double-layered structure in 1986 (taking phthalocyanine derivative as p-type semiconductor, taking perylene tetracarboxylic derivative as n-type semiconductor to form double-layered heterojunction structure, photoelectric conversion efficiency was about 1%), creating a new era of organic solar cell research. This study firstly introduced the concept of electron donor (p-type)/electron acceptor (n-type) organic double-layered heterojunction into solar cell, and explained the reason for high efficiency of cell is that induced dissociation efficiency of light-induced excitons at the double-layered heterojunction interface is relatively high.
In 1992, Heeger. A J and Yoshino K. found that under the light-induced conditions, rapid charge transfer occurred in the blended system taking conjugated photovoltaic polymer material as electron donor (Donor, abbr. D) and taking C60 as electron acceptor (Acceptor, abbr. A), and the rate of this process was far greater than the reverse process. The reason is that C60 is a large conjugated system, the electrons in the molecular orbital composed of 60 carbon atoms are in the delocalized state and therefore steadying external electrons. These findings make researches on photovoltaic polymer material solar cell become a new hotspot.
Heeger A. J. prepared bulk heterojunction solar cell by taking blended material system as active layer, which comprises photovoltaic polymer material MEH-PPV served as donor, C60 derivative PCBM served as acceptor. The photoelectric conversion efficiency of the cell was further enhanced due to a great expansion of the interface between donor and acceptor, and an improvement of dissociation efficiency of excitons. In the monochromatic light at 20 mW/cm2 and 430 nm, energy conversion efficiency reached 2.9%.
In order to further improve the energy conversion efficiency of the photovoltaic polymer material solar cell, it is crucially important to develop a new semiconductor photovoltaic polymer material with a narrow band gap and broad absorption region. The photovoltaic polymer material with narrow band gap in the prior art usually shows better absorption in the longwave range (for example, near-infrared), while shows weak absorption in partial visible region. In addition, photovoltaic polymeric material with narrow band gap has disadvantages of relatively low hole mobility and poor solubility. In recent years, Wong et al (Org. Lett. 2006, 8, 5033) have synthesized a novel coplanar chromophore thiophene-phenylene-thiophene (TPT) derivative, and reported that photovoltaic polymer material containing TPT had superior performances on hole mobility, band gap, high absorption coefficient and wide absorption range to the sunlight, however, hole mobility, band gap and absorption range of the TPT photovoltaic polymer material are not superior enough, and need further improvements.